WebRtc AudioTrack and VideoTrack

    WebRtc Stream 里封装了AudioTrack和VideoTrack Vector.    WebRtc的 A/V Track 其实就是 Video/Audio Source 和一些状态。   Track的状态定义在　MediaStreamTrack

template <typename T>
class MediaStreamTrack : public Notifier<T> {
 public:
  typedef typename T::TrackState TypedTrackState;

  std::string id() const override { return id_; }
  MediaStreamTrackInterface::TrackState state() const override {
    return state_;
  }
  bool enabled() const override { return enabled_; }
  bool set_enabled(bool enable) override {
    bool fire_on_change = (enable != enabled_);
    enabled_ = enable;
    if (fire_on_change) {
      Notifier<T>::FireOnChanged();
    }
    return fire_on_change;
  }

 protected:
  explicit MediaStreamTrack(const std::string& id)
      : enabled_(true), id_(id), state_(MediaStreamTrackInterface::kLive) {}

  bool set_state(MediaStreamTrackInterface::TrackState new_state) {
    bool fire_on_change = (state_ != new_state);
    state_ = new_state;
    if (fire_on_change)
      Notifier<T>::FireOnChanged();
    return true;
  }

 private:
  bool enabled_;
  std::string id_;
  MediaStreamTrackInterface::TrackState state_;
};

   一是Track自身是否可用，　enable　和 disable,  第二是TrackState,  这个跟在Track内的Audio/Video Source 相关，　视Source状态　分别置值为: kEnded 或者 kLive.  

Local Stream 中的Track通过 RtpSender 跟WebRtc Session相连接，　　　WebRtc Session 自己是AudioProvider/VideoProvider/DataProvider,  　可以接收数据。